The invention is directed to an improved method and circuit for peaking information-bearing signals, such as video signals developed in television receivers.
Analog signals used for the transmission of information may become degraded because of the limited bandwidth of their transmission channel. One such form of degradation occurs when the rise and fall times of an analog information signal become undesirably large.
For example, in television receivers an analog video signal is developed for intensity modulating the electron beam of a cathode ray tube. If the video signal does not have sharp leading and trailing edges (i.e., fast rise and fall times), the reproduced television image may not appear to be "crisp".
To develop a more crisp television image, it is conventional to "peak" the video signal so as to add preshoot and overshoot to it. The resultant faster rise and fall times of the video signal tend to provide a more pleasing television image.
One method which has been used to peak a video signal is to develop a peaking signal comprising the second derivative of the video signal and to subtract the peaking signal from the video signal itself. Although the resultant peaked video signal does have considerable pre-shoot and overshoot, any noise present in the original video signal becomes enhanced in the peaked video signal. Consequently, the television image becomes degraded. Accordingly, the amount of peaking which this method can practically provide is limited by the amount of noise present in the original video signal.
Two additional peaking schemes have been proposed for reducing the effect of noise in a peaked signal. In the television art, these schemes are referred to as "straight coring" and "non-linear amplifications". Both such schemes operate on the assumption that noise is primarily low level information. Based on that assumption, low level signals are removed from the peaking signal prior to subtracting the peaking signal from the original video signal. Of course, low level information signals are likewise indiscriminantly removed from the peaking signal along with the noise. Consequently, the peaked video signal develops an unnatural television image because of the lack of fine detail. An exemplary noise cancellation scheme employing coring of a television video signal is described in U.S. Pat. No. 3,995,108 to Morrison.
The problems noted above with prior peaking systems have rendered such systems less than entirely satisfactory, particularly because of their inability to reproduce television images having fine detail without a disproportionate amount of noise.